I am using Windows XP Professional for some specific software programs. Now I read in the latest German CT-Magazin that Microsoft will fix big security problems until 2014.
At microsoft.com is spoken about "mainstream" support (for SP3) until April 2012. Extended support until April 2017, but for "commercial customers" only (and what am I?).

Security updates will be available through the end of the Extended Support phase (five years of Mainstream Support plus five years of the Extended Support) at no additional cost for most products. Security updates will be posted on the Microsoft Update Web site during both the Mainstream and the Extended Support phase.

Microsoft won't continue supporting XP after another year. However, if there is a critical update needed to XP (such as to prevent a worm from spreading) then XP will then be updated. So in a sense, Microsoft will continue to support XP in a way until it's market share falls to such a number like 1%.